After migrating my files from High Sierra (10.13.6) to Mojave (10.14.2) using a Time Machine backup, many original image and video files inside the Photos Library disappeared from my hard drive. Actually I can still see them when I browse them in Photos.app. But exporting the original file does not work and the Photos.app complains that the file is missing. This probably means, that the thumbnails are still in the database and that Photos.app knows of the pictures and videos but can't find them on the file system. What is strange is that images and videos taken originally with my iPhone seem to be fine. Only images and videos sent to me via WhatsApp, then saved to Photos on my iPhone, and subsequently imported to Photos.app on my Mac, were lost during the migration to Mojave or possibly later due to a bug in Photos.app. My first thought was, that maybe something went wrong with the database migration from version 3 to version 4. In this case at least the original files should be in the Masters folder inside ~/Pictures/Photos Library.photoslibrary/. But I could not find any of the missing files there. Fortunately I haven't deleted the migrated account on my old Mac running the lates version fo High Sierra yet. There the Photos Library (version 3.0) is still intact and all the images and videos originating from WhatsApp on iPhone are also present in the Masters folder. Has anyone else experienced this kind of data loss after migrating a Photos Library from High Sierra to Mojave? Is this a know bug? Also I'm not sure if I should try to migrate the library again or leave it on High Sierra for the time being?